Discussion on:

IP subnetting made easy

What do you think is the most confusing thing about IP subnetting? Do these illustrations help demystify the concepts effectively?

For most its the Anding, for others its remembering what /24 means.
Some of those diagrams are usefull, though I'm not sure about the first one.

I've actually laid out VERY large and complex network using that first ruler. It's a visual tool for designing complex variable length subnets, supernets, and summarized subnets. That especially comes in handy for router, switch, and firewall engineers. Using the AND function and the /24 subnets is the basic stuff. The MCSE certifications don?t go that far in to subnet layout and design like the Cisco certification exams.

hey hows it going. do you know of additional areas where i can pick up more how to on subnetting. i did not get the ruler explanation

http://www.lanarchitect.net/Designs/SubnetRuler/SubnetRuler.html

The ruler let's you visually map and subnet without doing any math. See if the animation makes it a little easier.

I just wanted to express my gratitude for the subnet ruler. I am going to school to earn a Bachelors in IT and I am currently in a networking class. Having a visual aid like this sure does help to understand the subnetting guidelines. Thanks again for sharing.

OK, I'm still confused on this one. If 172.16.0.0 is a class B, shouldn't it be a /16?

I would have thought that 172.20.0.0/12 would run from 172.20.0.0 to 172.20.15.255. Where am I getting lost? The other examples make total sense to me.

Classes are just arbitrary borders and just adds confusion for students. Modern routers by default ignore the concept of classes and Cisco routers implement the default configuration of "IP classless" to ignore classes. Turning off that command and honoring classes can cause HUGE problems because the router will assuming anything starting with an IP below 127 is automatically a /8 which means the router (if it's running dynamic routing) will hijack massive numbers of routes. I've been the victim of this kind of sloppy configuration from affiliate companies who manage to screw up my routes big time because they were running in classful mode rather than classless mode.

Now when you see 172.16.0.0/12 (not 172.20.0.0/12), that is a private block of sixteen B-sized blocks. That goes all the way from 172.16.0.0 to 172.31.255.255. That second octet has 16 variations and each one of those represents a B-sized block of 2^16 addresses.

I would like to distribute this to some of our remote administrators

The article is open to the public and you can just send them the link.

Most people use the A class of the C class for sub-nets. I have not yet designed a network that needs an A class set of numbers. I also like to give the clients in each subnet plenty of room to expand, while still allowing them to have a reasonable subnet hierachy in place. Thus I tend to use the B class address numbers.

Take 172.16.x.x as the business network, it has a number of different locations or work groups, so I can use the 3rd octet to designate locations / work groups and then use the 4th octet for 254 pcs within that group. Thus all HQ units will be between 172.16.0.x and 172.16.9.x, gateway and system admin is 172.16.0.x, business admin is 172.16.1.x, sales 172.16.2.x etc - second facility is 172.16.10.x to 172.16.19.x, gateway and system admin is 172.16.10.x, business admin is 172.16.11.x etc.

Yes that means a lot of address go unused. But I once spent a nice two months reassigning sub-net addresses and adjusting routers and dns servers because the initial network only needed 20 clients in its largest sub-net with a total of 160 all up. It took 3 years for the business growth to trash the C class subnet that had been originally set up. Much of the time was spent travelling between facilities reprogramming the routers, dhcp servers and dns servers. Spent some time redseigning the system, then reprogrammed everything using both, got the new stuff running and went back around removing the old - all to ensure no problems with the day to day operation during the change over.

If you need more than this you can apply the same to the 10.x.x.x private address group. Much better to over allow for expansion than be too restrictive. And by using simple to remember digital octet groups you can readily tell what group and lcoation a particular IP address belongs to.

By applying the subnet at the last octet I can simplify the router instructions and make the subnet division easier to remember.

I use this tool any years ago.

http://www.radmin.com/download/ipcalc11.exe

Thanks for that useful article, George! It's sad to acknowledge that so many IT professionals today are unschooled in binary math -- once the first step in a computer-related education -- but even those of us who can rattle off powers of two in their sleep can benefit from your time-saving tips.

Thanks again. This one's a keeper.

http://www.lanarchitect.net/Designs/SubnetRuler/SubnetRuler.html

My animated ruler shows how it?s possible to map out a very large network. I might have to post a Visio with a vertical ruler as a useful download.

Oh, and please do note that I've issued a correction on Figure A and B so do download the newer version.

This is a very useful article on IP Subnetting and the first time I've ever seen anyone put all the information in one place. Great article.

George,

Excellent article - as another poster said, this one is a keeper. Even if one uses a subnet calculator program like SolarWinds it is still good to go back and review the basics.

The CCNA program I took at Sacramento City College drummed subnetting into our heads almost ad-nauseum, but I'm glad they did. I can almost (notice I said "almost") do subnetting in my head.

Also, thanks for making the article a downloadable PDF - good to keep on the laptop for reference.

Best Regards,
Bill Bowen

Sorry I had a mistake in Figure A and B. It was fixed on 6/3/2006. Please download the fixed version and discard the first version.

I haven't found the need for any special subnet calculators since I memorize numbers like 2, 4, 8, 16, 32 and 128, 192, 224, 240, 248, 252, 254, 255. Using the "AND" operator in Windows Calculator in decimal mode is also VERY useful and I never understood why that's usually not taught. I feel sorry for people when I see them doing binary conversions on tests.

The ruler allows you to allocate subnets without a calculator. You just make sure the edge tick marks are always taller than the tick marks in the middle and you know it?s a clean subnet.

Can you tell me (or point me to) how you use the "AND" operator in Windows Calculator in decimal mode? How does this help in subnetting?

It seems straight forward from a (hardware high-level)hardware (software low-level)standpoint.

I guest if the SW guys have start looking at protocol layers from a opposite or upside down view.

Anyway thanks for the cheat-sheet

George, although I've been subnetting for years, I've found that using rulers cuts my time in half. Even for complex installs, I can quickly layout the entire network, move things around until I'm happy, and not have to check and recheck my math.

Thanks for the tip.

I issued a correction over the weekend and it was fixed on Monday morning. I had errors in Figure A and B where I should have had a 0 in the second octet. If you've downloaded something, please re-download it. Sorry for my mistake!

I also forgot to mention that I have an animated version of the ruler here:
http://www.lanarchitect.net/Designs/SubnetRuler/SubnetRuler.html

The animated ruler shows how it?s possible to map out a very large network.

Very helpful indeed

Excellent article

Now I understand what /21 and /24 means after an IP address

Um, you're referring to the DECIMAL form of subnet addresses (re: 255.255.255.0), it is *NOT* base 256, it is base 10. It is the decimal format of a binary number. If it was base 256 it would have two possible values, 0 and 1, 0 being 0 and 1 being 256.

It scares me that you're teaching other people subnetting :/

Yes, a 4-digit base-256 number written with base-10 symbols and separated by periods. A 12 digit base-10 number would actually be 999.999.999.999 and last time I checked there is no such IP address. Even if we used Hex symbols like FF.FF.FF.FF (which is a valid way to write an IP address) to represent an IP address it would STILL be a 4-digit base-256.

The reason we use base-10 or base-16 symbols to represent a base-256 number is because no official symbols exist for base-256. But let's say that the symbol (+) had been chosen to represent the highest number in a base-256 system, then the largest IP address number would be (+).(+).(+).(+)

But all this is just arbitrary on how we use the period separator. An IP address could just as easily use 7 period separators for an 8-digit HEX number instead so the biggest IP address would look like "F.F.F.F.F.F.F.F". But instead we're using 3 separators making it "FF.FF.FF.FF" which makes it a 4-digit base-256 number.

If we were truly using a decimal system then IP addresses would run from 0 to 4294967296 though that would be extremely confusing.

I've personally have had programmers thank me that I'm the first to give the proper explaination that an IP address is a 4-digit base-256 number (written with base-10 symbols) and NOT a 12-digit base-10 number which would actually result in a maximum IP address of 999.999.999.999. This programmer never figured out why IP addresses stop at 255.255.255.255 and it finally made sense when I called it a base-256 number.

"It scares me that you're teaching other people subnetting :/"

You should to be careful about putting other people down when you clearly don't understand the math. I take a lot of pride in this particular article because it's used to teach IP subnetting in schools.

"If it was base 256 it would have two possible values, 0 and 1, 0 being 0 and 1 being 256."

YIKES!
Base-2 has 2 possible values from 0 to 1.
Base-10 has 10 possible values from 0 to 9.
Base-16 has 16 possible values from 0 to F.
Base-256 has 256 possible values from 0 to 255.

The reason I say 0 to 255 because there were no symbols created beyond base-16 so we're forced to use base-10 symbols to represent it.

This article has help me get out of the dark on this. But I'm still fuzzy on something. In looking over some test questions for an exam I saw a question with an IP of 192.168.0.100 that asks you to guess what subnet mask address it uses 255.255.255.0 or 255.255.240.0 with no other info given. The answer was 255.255.255.0 but I don't understand how they got this. I know the default subnet mask of a class C address is 255.255.255.0 but how do you know when you should use this or 240 etc etc

Thanks
qb

In general, any class C address is /24 or 255.255.255.0. Only supernetted class C address that are consolidated can be 255.255.240.0. 192.168.0.0 can actually have a mask of 255.255.0.0 if you wanted to. It's just that 255.255.255.0 is what's commonly used.

If that was a real question, it sucks. It's too vague and it gives multiple valid answers. You just have to pick the "best" answer.

You are really sharp with subnetting. Thanks for the education

I have to admit when I first saw the base 256 reference, I was also confused by it but now understand what you are saying.

Could we also refer to this as modulo 256?

This was made so easy that if a person is supposed to know about the subnetting, he will definetly understand that.

PLZ GIVE ME DETAILS OF MASK& UNMASK....

Im doing Cisco ccna1 version 4.0 currently and we have just being doing chapter 6 on subnetting and this is far better at explaining than cisco was..
They have mistakes in thier lab books.. and that can throw a person right off track.. so thank you for clearing this up for me once and for sure..

How do i determine the subnet mask when i only got an IP range? (ex: 207.46.200.0-207.46.207.255)

Excellent

For me this cheat sheet helps alot

http://krow.net/dict/subnet.html

its good

This is quite a nice solution for subnetting and I hope a lot of people will find it useful

very good article

very nice

Thank YOU! My exercise book and tutors have left my brain totally messed up trying to get to grips with this subject.
Phil.